In general, it is known to derive a binary (two-tone) image from a multi-tone image by a process of “thresholding.” In thresholding, pixels or picture elements of the image are assigned one of two tones (e.g., black or white), based on a determination of whether the initial tone of the pixel or picture element (e.g., a shade of gray) is above or below a chosen threshold (e.g., a chosen level of brightness).
Co-pending applications U.S. Ser. No. 09/422,584, filed Oct. 21, 1999, and U.S. Ser. No. 09/422,535, filed Oct. 21, 1999, disclose that features of interest within an image may be identified by thresholding.
U.S. Pat. No. 5,832,111 concerns a method for thresholding a video signal to produce a binary segmented video signal.
U.S. Pat. No. 6,199,986 concerns an automated method for measuring the wavefront aberration of an eye.
John C. Russ, The Image Processing Handbook, 2d ed. (CRC Press 1995) at pages 394-96 and 416-18, describes feature detection in a digitized image.